1. Field of the Invention
The present invention relates to an optical amplifier and a control method therefor.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying an optical signal or signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has already been developed to amplify signal light in a 1.55 μm band where the loss in a silica fiber is low. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 μm band or a 1.48 μm band, a gain band including a wavelength of 1.55 μm can be obtained.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
By using an optical amplifier as a linear repeater, the number of parts in the repeater can be greatly reduced as compared with the case of using a conventional regenerative repeater, thereby ensuring reliability and allowing a substantial cost reduction.
In the case of incorporating an optical amplifier into an optical fiber transmission system adopting WDM, various types of control for the optical amplifier are required to meet the requirement that the wavelength characteristic of gain must be kept constant and to prevent the waveform degradation due to the nonlinear effects in an optical fiber transmission line.
For example, the wavelength characteristic of gain in an EDFA changes according to the gain determined by a pumping condition, so that AGC (automatic gain control) is performed by giving a constant gain to an input to obtain an output. In this case, the output changes with a change in the input under the constant gain.
In an optical amplifier, a higher signal output is desirable from the viewpoint of an S/N. However, it cannot be said that this is generally true, in consideration of the wavelength degradation due to the nonlinear effects in an optical fiber transmission line and the input dynamic range at a receiving end. That is, it is required to perform ALC (automatic level control) so that the output from the optical amplifier becomes constant in a predetermined range.
There has been proposed an optical amplifier including first and second optical amplifying units and a variable optical attenuator connected between the first and second optical amplifying units as an optimum configuration for performing both AGC and ALC. In this configuration, AGC is performed in each of the first and second optical amplifying units, and ALC is performed by the variable optical attenuator.
This configuration has been proposed for the following reasons. The first reason is that if the variable optical attenuator for ALC is arranged on the front stage, there is a disadvantage from the viewpoint of optimization of the NF (Noise Figure) in the optical amplifier as a whole. The second reason is that if the variable optical attenuator for ALC is arranged on the rear stage, it is necessary to obtain a higher signal output power in the optical amplifying unit for AGC just upstream of the variable optical attenuator, so as to meet the requirement that a given signal output power from the optical amplifier must be ensured as a whole, resulting in a disadvantage from the viewpoint of reduction in power consumption of a laser diode as a pumping source.
In the above configuration suitable for both AGC and ALC, it is necessary to independently perform AGC in each of the first and second optical amplifying units, causing a problem that the configuration of the optical amplifier becomes complicated.
Further, in the case of using this optical amplifier in a system adopting WDM, there arises another problem that when the number of WDM channels changes, the control of the variable optical attenuator for ALC is complicated. More specifically, in the case of performing ALC in amplifying WDM signal light, the control is performed so that the total output power from the variable optical attenuator becomes constant. Accordingly, when the number of wavelength channels of the WDM signal light changes during operation of the system, a target value for the control of the variable optical attenuator is changed.
This target value for the control of the variable optical attenuator is generally transmitted from a supervisory control device provided upstream of the variable optical attenuator, so that a troublesome supervisory operation is required in response to a change in number of wavelength channels in the system. While the attenuation of the variable optical attenuator is temporarily fixed in response to a change in number of wavelength channels, it is necessary to carry out such a operation that the target value is updated to a power target value according to the change in number of wavelength channels in the condition where an ALC loop is open, and the ALC loop is thereafter closed. Accordingly, there is a possibility that the attenuation of the variable optical attenuator may vary during this operation. Further, since AGC is continuously performed in each of the first and second optical amplifying units, there is a possibility that the output power per wavelength channel may vary in switching the control of the variable optical attenuator.